Minireview: pancreatic progenitor cells--recent studies.

Past studies of pancreatic progenitor cell biology relied mostly on histological analyses. Recent studies, using genetic labeling and tracing of progenitors, direct single cell analyses, colony assays, and enrichment of the minor population of progenitor cells through the use of cell surface markers, have strongly suggested that pancreatic progenitor cells with various frequency and lineage potentials, including the multipotent progenitors that give rise to endocrine, exocrine, and duct cells, exist in the developing and adult pancreas. In this review, it is therefore proposed that pancreatic progenitor cells may be organized in a hierarchy, in which the most primitive pan-pancreatic multipotent progenitors are at the top and rare, and the monopotent progenitors are at the bottom and abundant. This model may explain why only drastic injuries lead to effective activation of the progenitor cell compartment of the higher hierarchy, whereas under steady state, pregnancy, and milder injuries, recruitment of preexisting mature cells or their immediate monopotent progenitors could be sufficient to restore metabolic homeostasis. It is also proposed that the morphologically defined ductal cells are likely to be functionally heterogeneous and that endocrine progenitor cell activity should be determined based on functional analyses rather than histological locations.